An optical fiber, which comprises a core region with a predetermined refractive index and a cladding region, provided at the outer periphery of the core region, haying a refractive index lower than the core region, is fabricated by drawing an optical fiber preform with the same refractive index distribution. The core region is generally doped with GeO2 in order to increase the refractive index of the core region but the core region is also sometimes doped with Al2O3.
In comparison with an optical fiber whose core region is doped with GeO2 (hereinafter referred to as Ge-doped optical fiber), an optical fiber whose core region is doped with Al2O3 (hereinafter referred to as Al-doped optical fiber) has stable raw material costs because the Al-element occurs naturally in abundance. Furthermore, in Al-doped optical fiber, there is the possibility of a contribution to reduced loss on account of its small Rayleigh scattering coefficient. Patent Document 1 discloses a technology for fabricating an Al-doped optical fiber.
However, Al-doped optical fiber has hitherto not been put to practical use. This is because it has proven actually extremely difficult to reduce transmission loss (Non-patent Document 1). Causes of the transmission loss that occurs with Al-doped optical fibers include the crystallization of Al2O3 and the incorporation of impurities.    [Patent Document 1] U.S. Pat. No. 4,826,288    [Non-patent Document 1] P. C. Schultz, Journal of the American Ceramic Society, Vol. 57, No. 7, pages 309 to 313 (1974)